1. Field of the Invention
The present invention generally relates to the field of optical data communications, and more specifically to an electro-optic modulator for operation at microwave frequencies with efficient input coupling and smooth wideband frequency response.
2. Description of the Related Art
Transmission of data using optical carriers enables very high bandwidths and numbers of multiplexed channels with low signal loss and distortion. A coherent laser light beam is amplitude modulated with a data signal, and propagates to a remote receiver either directly through the atmosphere, or via a system of optical fibers and repeaters. The light beam may advantageously be modulated with electrical signals in the microwave frequency range using an electro-optic modulator such as a Mach-Zehnder modulator or optical coupler.
An electro-optic modulator based on a Mach-Zehnder interferometer generally includes a monolithic substrate formed of an electro-optic material such as lithium niobate (LiNbO.sub.3) An optical waveguide is formed in the substrate having two arms or branches which extend generally in parallel with each other. The index of refraction of the material in the waveguide is higher than the index of refraction of the material of the substrate.
In the basic Mach-Zehnder modulator, the branches have equal lengths. In the absence of an applied electrical bias voltage, an input optical or light beam produced by a laser or the like applied to the waveguide divides equally between the branches. The optical signals propagating through the branches recombine at the optical output of the waveguide in phase with each other, such that their amplitudes are additive and an optical output signal which is essentially similar to the optical input signal appears at the output of the waveguide.
Application of a predetermined electrical bias voltage differential between the waveguide branches causes a differential in the indices of refraction of the two branches due to the electro-optic effect, with a corresponding differential in the effective optical lengths of the branches. At a bias voltage known in the art as V.pi., the effective optical lengths have varied to such an extent that the optical signals emerging from the branches are 180.degree. out of phase with each other. The amplitudes of the signals combine subtractively, canceling each other out, such that a zero output is produced at the optical output. For most optical communication applications, it is desirable to bias the modulator at a voltage V.pi./2, which produces linear operation.
Some of the problems which have limited the high frequency broadband application of Mach-Zehnder electro-optic modulators are specific to the modulator electrodes. These include impedance mismatch losses at the modulator input, and the presence of spurious resonances at the modulator's frequency response.
A Mach-Zehnder electro-optic modulator of the type to which the present invention relates is described in a paper entitled "Traveling-wave 1.3-.mu.m interferometer modulator with high bandwidth, low drive power, and low loss", by D. Dolfi, in Applied Optics, vol. 25, no. 15, August 1986, pp. 2479-2480. The device as disclosed includes a modulation signal source having a 50.OMEGA. output impedance which applies an electrical modulation input signal to a coplanar waveguide structure in the modulator with an input impedance of 22.OMEGA.. The reflection loss at the input due to the impedance mismatch reduced the effective drive voltage to 61% of the source voltage. An attempt to eliminate the reflection and minimize ripples in the frequency response included connecting a 28.OMEGA. series resistance between the source and modulator. However, this reduced the effective drive voltage even further to 44% of the source voltage.
A similar device is disclosed in a paper entitled "Microwave integrated optical modulator", by P. Cross, Applied Physics Letters 44(5), Mar. 1, 1984, pp. 486-488. The reflection loss was essentially similar to that reported by Dolfi. Connection of a resistive network between the source and modulator to minimize ripples in the response reduced the effective drive voltage from 63% to 27%. The device further suffered from spurious resonant modes in the frequency response resulting from an undesired microstrip mode existing between the hot lead of the coplanar line and the metal package.
Another similar device is disclosed in a paper entitled "Traveling-wave electrooptic modulator", by C. Gee et al, in Applied Optics, vol. 22, no. 13, July 1983, pp. 2034-2037. A prominent feature of the modulator was a pronounced resonance at 5.5 GHz generated in the device package.
A further related device is disclosed in a paper entitled "10 GHz Bandwidth Traveling-Wave LiNbO.sub.3 Optical Waveguide Modulator", by M. Izutsu et al, in IEEE Journal of Quantum Electronics, vol. QE-14, no. 6, June 1978, pp. 394-395. The device is a phase modulator, rather than an amplitude modulator, and achieved smooth frequency response using a symmetrical electrode configuration as a microwave waveguide.